Method for Preserving Foodstuffs

ABSTRACT

A method for preserving foodstuffs, in which the foodstuffs are heated in the moist state in a container suitable for transport and storage with ventilation openings. The foodstuffs are heated for a defined period by a microwave, for at least such a time as hot steam forms in the container and exits through the ventilation openings. Gas is injected into the container after the heating process for at least partial compensation of the pressure drop in the container.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of InternationalApplication No. PCT/CH2006/000063, filed on Feb. 1, 2006, which claimspriority of Swiss application number 00219/05, filed on Feb. 10, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for stabilizing foods and inparticular pasteurizing foods in which the foods in a moist state insidea container suitable as a shipping and retail package and having a ventopening are heated by microwaves for a limited period of time but atleast long enough so that hot steam forms in the container and escapesthrough the vent opening.

For further use for shipping and retail, the vent opening of thecontainer must be sealed, even if it is very small, to preventcontamination of the foodstuffs with microorganisms and possibly evenleakage of fluid out of the container.

2. Description of the Prior Art

A method of the aforementioned type is known from WO 2004/045985 A1, forexample. In this method, a container having a flexible bottom and aflexible cover film is used, with a valve being arranged in it as a ventopening. The valve opens automatically during heating under theinfluence of the excess pressure generated in the container and therebyallows steam to escape. Even shortly after heating, with the onset ofcooling and the resulting drop in pressure, the valve automaticallycloses again. Subsequently, there is a further drastic drop in pressurewith further cooling and due to the condensation of the vapor whichpractically creates a vacuum in the container. The flexible bottom andthe flexible cover film of the container bulge inward under theinfluence of the vacuum and thereby securely enclose the foodstuffspresent in the container between them.

The same effect is utilized as described in WO 00/03605 A1 and in WO03/087993 A1 to achieve a tight and secure sheathing of the foodstuffswith an especially flexible cover film.

EP 1 076 012 A1 describes a preparation method for foods in which thefoodstuffs in a moist state are heated with microwaves in a containerwith a partially flexible cover film and an excess pressure valve for alimited period of time, but at least long enough until steam forms inthe container and escapes through an excess pressure valve. This knownmethod is available and is suitable in particular for bringing foods ina raw fresh state on the market in a container and only preparing themin the container in a microwave oven by the end user immediately beforeconsuming them. Due to microwave steam preparation under elevatedpressure, the foodstuffs are prepared in an extremely gentle manner fromtheir raw state within only a few minutes, whereby their nutritionalcontent, their consistency, color and natural taste are largelypreserved.

However, as part of this usage, it would be unfavorable for a vacuum todevelop in the container as described above. The foodstuffs which havejust been prepared freshly would be crushed by the covering film bulginginward and liquid might also be pressed out of them. This would destroythe very attractive appearance of the foodstuffs, which has just beenachieved. Furthermore, the container as such would be deformed in anunattractive manner. EP 1 076 012 A1 solves this problem by a specialdesign of the excess pressure valve. The valve is altered during theheating process so that it can no longer close again or at least can nolonger close immediately. This prevents the development of a vacuum inthe container. The destruction of the valve in the method according toEP 1 076 012 A1 does not constitute a problem because the foodstuffshere are intended for consumption immediately after being prepared. Thecontainer is also designed only for a single use and then is no longerneeded. To render the foodstuffs stable through heating at themanufacturer's before the shipping and retail phase, the methodaccording to EP 1 076 012 A1 would not be suitable, however, because ofthe excess pressure valve being destroyed there.

To at least partially avoid the vacuum problem, EP 1 359 097 A1 proposesthat in a corresponding method, the microwave treatment inpasteurization should be of such dimensions that the resulting vacuum isno longer great enough to destroy or completely deform the container. Inpractice, however, this means that for effective pasteurization at alower temperature, heating must be continued for a longer period of timethan in EP 1 076 012 A1 by a factor of at least 10, so that theadvantages achieved with the preparation method according to EP 1 076012 A1 and through the short preparation times would be virtually lostagain completely.

Patches are also known in which a vent opening without a valve isactively closed again after heating, e.g., with an adhesive patch. Inthis case, the sealing of the container could basically wait until thecontainer and the foods have cooled sufficiently and an equalization ofpressure through the vent opening has been established. However, thetime required for this would be substantial and would be a burden on aneffective production rate. Due to the air flowing into the containerduring this pressure equalization phase, microorganisms could also enterthe container and impair the stability of the foodstuffs. To have theequalization of pressure performed under a sterile protective gasatmosphere would at the very least be complex in terms of equipment.

It is possible to provide for the foodstuffs to be heated again by theend user before the foods are consumed and again for this to be done inthe container, optionally with the use of microwave heating. Since thefoodstuffs have at least partially been pre-prepared through the priorheating to make them stable and/or pasteurize them, simply warming themhere is usually sufficient. In this context we also speak ofregeneration in contrast with preparation from the raw state. However anexcess pressure situation might nevertheless also develop in thecontainer. If there is a valve that opens automatically, it may play arole in pressure release. If a vent opening has been sealed again withan adhesive patch after stabilizing the contents, then an automaticpressure release may no longer be available. Accordingly, the end useris often required to puncture the container before heating it, forexample, or expose the sealed vent opening again, e.g., by removing theadhesive patch applied to it. However, adhesive patches which becomedetached with heating and thereby automatically release the vent openingagain have already become known.

Finally, EP 1 359 097 A1 also cites in the introduction other methods ofthe state of the art, including heating containers without a ventopening under a backpressure, but they are not very economical and theyrequire complicated and expensive plant technology.

SUMMARY OF THE PRESENT INVENTION

The object of the present invention is to provide a method ofstabilizing and in particular pasteurizing foodstuffs of the typedefined in the introduction with which the advantages of the preparationmethod known from EP 1 076 012 A1 can also be achieved by themanufacturer and thus also prior to the shipping and retail phases.

This object is achieved according to the present invention, namely by amethod for stabilizing foodstuffs in which the foodstuffs are in a moiststate inside a container suitable as a shipping and retail package andhaving a vent opening. The foodstuffs are heated with microwaves for alimited period of time but at least until hot steam has formed in thecontainer and escapes through the vent opening. A gas is injected intothe container for at least partial compensation of the pressure drop inthe container after the end of the heating. The core of the inventionthus consists of the fact that a gas is injected into the container forat least partial compensation of the pressure drop in the containerafter the end of heating.

Injection of the gas may be performed easily and rapidly in only a fewseconds and under a pressure of a few bar, e.g., using a cannula. Tothis end, the cannula may be inserted through the vent opening. However,the cannula is preferably used to puncture a wall of the container, andthe container is flushed with the gas while expelling vapor through thevent opening. It is thermally advantageous if the injection gas isprecooled.

A gas having little or no oxygen content is also preferred as theinjection gas with which, in addition to the steam, the oxygen which isstill present in the container but is especially harmful for thestability of the foodstuffs is flushed out of the container. Especiallygood stabilities can be achieved when an inert gas such as nitrogen oran antibacterial gas such as CO₂ or a mixture of these two gases is usedas the injection gas.

The seal on the puncture hole optionally created in the container wallduring injection of the gas can be accomplished easily, e.g., byapplying an adhesive patch to the container wall.

By applying an adhesive patch, the vent opening could be sealed if avalve that opens automatically with an excess pressure and closes againautomatically when the excess pressure drops is not used for this ventopening. However, even in this case, it may be advantageous, e.g., foran absolutely secure closure to also cover the valve with an adhesivepatch as well. If automatic opening of the container to release pressurein regeneration is necessary, then an adhesive patch that is releasedunder the influence of heat may be used.

In comparison with an adhesive patch that is released under theinfluence of heat, however, a valve still has the advantage that itallows outgassing of the foods, for example, during storage, shippingand/or the retail phase. After a conventional pasteurization, theenzymatic activity of the food is usually only weakly pronounced as acause of the evolution of gas.

If both openings are to be covered with an adhesive patch, then one andthe same adhesive patch is advantageously used. In this case, the twoopenings should not be situated too far apart.

For use within the scope of the present invention, the valves describedin EP 1 076 012 A1 or EP 1 359 097 A1 are suitable in particular,although they should not be destroyed by the thermal stresses that occurduring heating in particular. These valves have a suitable flowresistance, which is also stable due to a dimensionally stable valvebody and does not change under the resulting loads.

A defined and approximately constant flow resistance is important sothat the result of the stabilization process can be predictable andrepeatable in a narrow tolerance range. Even if only a simple ventopening without a valve function is used, it should have a defined crosssection, preferably changing as little as possible.

Within the scope of the inventive method, it is possible and evenpreferable to use raw fresh foods. In this case, a temperature of atleast 100° C. in the vapor and/or 80° C. in the core zone of thefoodstuffs should be created for their preparation and adequatestabilization. With the microwaves used according to the presentinvention, this can be achieved within 1-7 minutes, depending on theweight of the product.

In order for the container to be reliably usable for this purpose, itshould be capable of withstanding temperatures up to 150° C. duringheating and/or an excess pressure of up to 1.5 bar.

In order for sufficiently high temperatures and/or pressures to bereachable during heating at all, the flow cross section of the ventopening must not be too large. The resulting vapor must stagnate in thecontainer to a certain degree. This is achieved, for example, if thevent opening has a flow cross section of 1-20 mm², preferably 3-7 mm².

The containers known from EP 1 076 012 A1 comply with the aforementionedrequirements and can therefore also be used within the scope of thepresent invention. Their cover film has a thickness between 40 and 200μm. It would also be conceivable to use bag-like containers madeentirely of such a plastic film or containers having a solid plasticshell and rigid cover. The latter can be manufactured from hard films orby injection molding and have a wall thickness in the range between 600and 3000 μm. The package sizes may vary between 10 g for smallindividual portions and 5000 g for large-scale distributor drums.

The foodstuffs could be pre-prepared and/or blanched entirely orpartially before being added to the containers.

By injection of a gas having a low oxygen content, as explained above,it is possible for the oxygen which is harmful for the stability of thefoodstuffs to be removed, at least partially from the container. Inaddition, this reduction in the oxygen content can be supported bypackaging the foodstuffs already under a reduced pressure in thecontainer and/or in a protective gas atmosphere having a low oxygencontent.

As is customary, the containers with the foods that have been stabilizedaccording to this invention are sent to a refrigeration chain withrefrigeration temperatures between 1° C. and 8° C. for the subsequentstorage, shipping and retail phases.

According to another preferred embodiment of the invention, heating isperformed as the container passes through a microwave tunnel or amicrowave chamber.

The microwave tunnel is preferably followed directly by an injectionstation in which the gas is also injected in its run through thestation. Preferably the adhesive patch(es) is/are applied immediatelyafter the gas injection in the injection station although essentially aseparate station could be provided for this.

For conveyance of the container through the microwave tunnel and throughthe injection station, a conveyance mechanism is advantageously used. Inparticular, this may be the same conveyor system. To improve theeconomic aspect of the method, the containers may be conveyed in severalrows side-by-side through the microwave tunnel and/or the injectionstation.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is explained in greater detail below on the basisof exemplary embodiments in conjunction with the drawings, in which:

FIG. 1 a shows a container having a valve as a vent opening andcontaining foods prior to the latter being stabilized, this containerbeing suitable for use within the context of the inventive method,

FIG. 1 b shows the valve from FIG. 1 a in an enlarged diagram,

FIG. 2 a shows the container from FIG. 1 during heating by twomicrowaves,

FIG. 2 b shows the valve from FIG. 2 a in an enlarged diagram,

FIG. 3 shows the same container as in FIG. 1 or 2 after recooling andcondensation of the vapor formed during heating according to the stateof the art,

FIG. 4 shows the injection of a gas via a cannula into the containerafter heating according to FIG. 2,

FIG. 4 a shows a preferred embodiment of the cannula tip in an enlargeddiagram,

FIG. 5 shows the sealing of the puncture hole caused by the cannulaaccording to FIG. 4 by an adhesive patch,

FIG. 6 shows the container from the preceding figures with the foodsstabilized according to this invention,

FIG. 7 shows another container suitable for use within the scope of theinventive method and having a simple vent opening and foods in the phaseaccording to FIG. 4 whereby the gas is injected through the vent openingby a cannula,

FIG. 8 shows the container from FIG. 7 after retraction of the cannulaand closure of the vent opening with an adhesive patch,

FIG. 9 shows a container with foods according to FIG. 8 but with thecannula inserted next to the vent opening and with the adhesive patchfor sealing the puncture hole, shown on an enlarged scale accordingly,

FIG. 10 shows a device for automated implementation of the inventivemethod.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a shell-shaped container 10 made of plastic with aperipheral edge 11 onto which is welded a cover film 12 also around theedge. The cover film may be a plastic film, optionally in severallayers, with a thickness in the range between 40 and 220 μm. A ventopening 20 is provided in the cover film 12 through which a valve 21that opens automatically under excess pressure and closes againautomatically after the pressure has dropped is formed, as is knownessentially from EP 1 359 097 A1. Foodstuffs 30, which still have acertain inherent moisture content and are still in a raw fresh state,are contained in the container 10.

FIG. 1 b shows the valve 21 on an enlarged scale. It is made of plasticand has a dimensionally stable valve body 22 in which there are multipleopenings 23. The openings 23 are sealed by an elastic membrane 24 whichis held in position by a web 25. The sealing effect of the membrane issupported by sealing oil which is present especially in a groove 26.

FIG. 2 a shows the container 10 during heating with microwaves M tostabilize the foodstuffs 30, with steam D being formed from the moisturepresent in the foodstuffs 30 and causing an excess pressure P> in thecontainer 10. Under the influence of this excess pressure P>, the valve21 has opened so that steam D can flow out of the container 10.Furthermore, the cover film 12 has bulged up under the influence of theexcess pressure P>.

FIG. 2 b shows the valve 21 from FIG. 2 a in the opened state with themembrane 24 having an elastic bulge.

If the container 10 of FIG. 2 a were to be left to itself after beingheated, then the valve 21 would close again at some point with the onsetof recooling, a decline in vapor formation and another drop in pressureand then a considerable vacuum P< would develop in the container 10especially due to the resulting condensation of the vapor that is stillpresent, as is known from the state of the art cited in theintroduction, for example. FIG. 3 shows a container 10 having a coverfilm 12 bulging inward under such a vacuum P<.

The present invention counteracts the development of such a reducedpressure by injection of a gas G into the container 10. This isaccomplished in FIG. 4 by a cannula 40 with which the cover film 12 (asthe thinnest container wall) is punctured once. The container 10 ispreferably even flushed with the gas G, expelling steam D through thevalve 21. The gas G is injected, e.g., with an excess pressure of 1-5bar, preferably 3 bar. In this case it is sufficient to flush thecontainer for 1-20 seconds, preferably for approximately 8 seconds.

Preferably, a mixture of nitrogen and CO₂ in a volume ratio of80:20-70:30 is used as the gas G, where the gas G is preferablyprecooled to a temperature of less than 12° C.

To prevent a reduced pressure P< from being able to develop eventemporarily in the container 10, the injection of the gas G must beperformed relatively rapidly but preferably within 150 seconds after theend of heating.

FIG. 4 a shows a preferred embodiment of the tip of a cannula 40 in anenlarged diagram which shows the actual tip as a rectangle provided withsharp polished edges to simplify insertion. Four outflow openings 41 forthe gas G are distributed radially over the circumference above thepolished area. An especially uniform and effective flushing of thecontainer with gas is achieved with this embodiment.

FIG. 5 shows the container 10 after injection of the gas G, the cannula40 having already been retracted back out of the container 10. Thenapproximately ambient pressure prevails in the container, this beingdiscernible by the flat shape of the cover film 12. Excess pressuregenerated by the injection of the gas into the container 10 could bedissipated through the valve 21 and/or through the puncture hole 13created by the cannula 40 in the cover film 12. The valve 21 was alsoable to close again and the pressure was successfully equalized.

In order for the valve 21 to be able to close tightly again, theabovementioned sealing oil must at least not be flushed out completelyby the hot steam flowing through the valve 21 during heating in thevalve design illustrated in FIG. 1 b. It is advantageous here if thesealing oil is a silicone oil and/or has a viscosity (at 20° C.) of1000-20,000 Centipoise and/or is present in an amount of 4-8 mg.

To completely reclose the container 10, the puncture hole 13 in thecover film 12 is also sealed by applying an adhesive patch 50, as alsodepicted in FIG. 5.

For applying the adhesive patch 50, a stamp 60 which receives theadhesive patch 50 in its position shown in FIG. 4, e.g., from a labeldispenser (not shown), is used in FIG. 5 and holds the adhesive patch,e.g., by suction, until it is applied to the container 10.

The stamp 60 executes a pivoting movement, which is advantageous in thatthe container 10 need not be moved to apply the adhesive patch 50 afterretraction (linear here) of the cannula 40. Application of the adhesivepatch 50 immediately after retraction of the cannula 40 is thereforepossible and is also ensured if the container is conveyed on a conveyorbelt of a conveyor device, for example, and if the conveyor belt comesto a standstill. Alternatively or additionally, the cannulas 40 could ofcourse also be guided in a nonlinear manner.

The adhesive patch 50 should preferably also be applied immediatelyafter retracting the cannula so that no microorganisms can enter thecontainer from the outside. A period of time of 60 seconds between theinjection of the gas and the application of the adhesive patch istolerable, however.

FIG. 6 shows the container 10 with the foodstuffs 30 that have beenstabilized according to this invention in the gas atmosphere G underambient pressure and the adhesive patch 50 which is attached here. Inthis form, the container is suitable as a shipping and retail packagingand is preferably sent to a conventional refrigeration chain withrefrigeration temperatures in the range between 1 and 8° C., forexample.

FIGS. 7-9 show an alternative embodiment with a container 10 which,instead of being provided with a valve, has a vent opening 20 of asimple design in which the cover film 12 is provided. However, the ventopening 20, like the valve 21, has a dimensionally stable body 27 with adefined opening 28 and therefore with a defined flow resistance which atleast has not changed significantly under the stresses that occur duringheating. After injection of the gas G, the opening 28 must be activelyclosed, which may again be accomplished with an adhesive patch.

In the example in FIG. 7, the gas G is injected directly through theopening 28 into the container 10 with a cannula 40, so that noadditional injection hole is formed. In this case, it is sufficient toapply a single adhesive patch 51 over the opening 28 to seal thecontainer 10. However, one disadvantage with this variant is that thecontainer 10 cannot be flushed so effectively with the gas.

However, it is also possible to inject the gas into the container 10 ata point that is not too great a distance away from the vent opening 20and/or 28. In this case, flushing can be more effective and thecontainer 10 can still be sealed by applying just one adhesive patch 52,optionally somewhat larger, as illustrated in FIG. 9.

FIG. 10 shows a device for automated performance of the inventive methodwith a conveyor device 70 having a conveyor belt 71 which is passedthrough a microwave tunnel 80 and an injection station 90. A pluralityof containers 10, as shown here, can be conveyed on the conveyor belt 71through the microwave tunnel and through the injection station. Theheating of the containers 10 and the foodstuffs contained in thecontainers takes place in microwave tunnel 80 by means of microwaves,and in the injection station the gas injection described above isperformed and the adhesive patch(s) is/are applied. The device from FIG.10 may also be designed so that several containers are transportedside-by-side in several rows through the microwave tunnel 80 and theinjection station 90.

What has been described above are preferred aspects of the presentinvention. It is of course not possible to describe every conceivablecombination of components or methodologies for purposes of describingthe present invention, but one of ordinary skill in the art willrecognize that many further combinations and permutations of the presentinvention are possible. Accordingly, the present invention is intendedto embrace all such alterations, combinations, modifications, andvariations that fall within the spirit and scope of the appended claims.

LIST REFERENCE NUMERALS

-   10 container-   11 edge of container-   12 cover film-   13 puncture hole-   20 vent opening-   21 valve-   22 valve body-   23 openings in the valve body-   24 membrane-   25 web-   26 groove for sealing oil-   27 dimensionally stable body-   28 opening in 27-   30 foodstuff-   40 cannula-   41 outflow openings-   50 adhesive patch-   51 adhesive patch-   52 adhesive patch-   60 stamp-   70 conveyor device-   71 conveyor belt-   80 microwave tunnel-   90 injection station-   D steam-   G gas-   P> excess pressure-   P< reduced pressure

1. A method for stabilizing foodstuffs in which the foodstuffs are in amoist state inside a container, having a wall, suitable as a shippingand retail package, the packing having a vent opening, said methodcomprising the steps of: heating the foodstuffs with microwaves for alimited period of time, but at least until hot steam has formed in thecontainer and escapes through the vent opening; and injecting a gas intothe container for at least partial compensation of the pressure drop inthe container after the end of the step of heating the foodstuffs. 2.The method according to claim 1, wherein the gas is at least one of anoxygen-free gas, nitrogen and CO₂.
 3. The method according to claim 1,wherein said step of injecting the gas comprises injecting the gas at anexcess pressure of 1-5 bar.
 4. The method according to claim 1,comprising the step of precooling the gas to be injected to atemperature of less than 12° C.
 5. The method according to claim 1,wherein said step of injecting the gas comprises injecting the gaswithin 150 seconds after the end of said heating step.
 6. The methodaccording to claim 1, wherein said step of injecting the gas comprisesinjecting the gas for 1-20 seconds.
 7. The method according to claim 1,wherein said step of injecting the gas into the container comprisesflushing the container with the gas, to expel steam through the ventopening.
 8. The method according to claim 1, wherein said step ofinjecting the gas comprises using a cannula to inject the gas.
 9. Themethod according to claim 8, wherein said step of injecting the gascomprises puncturing the wall of the container with the cannula to forma puncture hole.
 10. The method according to claim 9, wherein the wallof the container that is punctured with the cannula comprises a plasticfilm having a thickness in the range between 40 and 220 μm.
 11. Themethod according to claim 9, further comprising the steps of: retractingthe cannula; and sealing the puncture hole in the container wall afterretracting the cannula.
 12. The method according to claim 11, whereinsaid step of seal the puncture hole comprises applying an adhesive patchto the container wall.
 13. The method according to claim 1, furthercomprising the step of sealing the vent opening after injecting the gas.14. The method according to claim 13, wherein said vent openingcomprises a valve that opens automatically under excess pressure andcloses again automatically after the excess pressure drops.
 15. Themethod according to claim 14, further comprising the step of sealingsaid valve with an oil as a sealing agent, wherein the oil is at leastone of a silicone oil, has a viscosity of 1000-20,000 Centipoise at 20°C. and is present in an amount of 4-8 mg.
 16. The method according toclaim 13, wherein said step of sealing the vent opening comprisesapplying an adhesive patch.
 17. The method according to claim 16,wherein the same adhesive patch is used for sealing the puncture holeand the vent opening.
 18. The method according to claim 12, wherein theadhesive patch is applied within 60 seconds after injection of the gas.19. The method according to claim 1, wherein at least one of atemperature of at least 100° C. is created in the steam and 80° C. inthe core zone of the foodstuff within 7 minutes during heating in thecontainer.
 20. The method according to claim 1, wherein said step ofheating the foodstuffs comprises passing the container through a deviceselected from the group consisting of a microwave tunnel and a microwavechamber.
 21. The method according to claim 1, further comprising thesteps of: passing the container through an injection station; andinjecting the gas during said step of passing the container through theinjection station.
 22. The method according to claim 21, wherein saidstep of applying the adhesive patch occurs during the step of passingthe container through the injection station.
 23. The method according toclaim 21, comprising the step of conveying the container on a conveyorsystem through the microwave tunnel and the injection station.
 24. Themethod according to claim 23, wherein said conveyor system comprises aconveyor device for conveying multiple containers simultaneously inseveral rows side-by-side.
 25. The method according to claim 1, whereinthe foodstuffs are packaged in the container in a raw fresh state. 26.The method according to claim 1, wherein the foodstuffs are packaged inthe container under reduced pressure and/or in a protective gasatmosphere having a low oxygen content.
 27. The method according toclaim 1, further comprising the step of supplying the container to arefrigeration chain having refrigeration temperatures between 1 and 8°C. after injection of the gas.
 28. The method according to claim 1,wherein said container withstands temperatures up to 150° C. duringheating.
 29. The method according to claim 1, wherein said containerwithstands an excess of up to 1.5 bar absolute during heating.
 30. Themethod according to claim 1, wherein said vent opening of said containerhas a defined flow resistance which does not change even under thestresses occurring during heating and has a flow cross section of 1-20mm².
 31. The method according to claim 2, wherein the gas is a mixtureof nitrogen and CO₂ in a volume ratio of 80:20 to 70:30.
 32. The methodaccording to claim 3, wherein said step of injecting the gas comprisesinjecting the gas at an excess pressure of 3 bar.
 33. The methodaccording to claim 6, wherein said step of injecting the gas comprisesinjecting the gas 8 seconds.
 34. The method according to claim 10,wherein said a plastic film comprises multiple layers.
 35. The methodaccording to claim 30, wherein said vent opening has a flow crosssection of 3-7 mm².